1. Field of the Invention
The present disclosure relates generally to a power transfer system and in particular, a power transfer system for electric vehicles and other related power transfer applications.
2. Related Art
Electronic devices such as laptops, cell phones, smart phones, smart devices, smart watches, tablets, MP3 players, digital media players generally require batteries and in some cases employ wireless power transfer (WPT) systems in order to charge the batteries of the devices. Charging systems for electric and/or hybrid vehicles may also utilize wireless power transfer (WPT) systems. Given the increasing demand for hybrid and electric vehicles as well as increased use of electronic devices, automotive companies and electronic device manufacturers are each motivated to design and manufacture improved high power WPT systems for vehicles and non-vehicle electronic devices respectively.
Many WPT systems utilize a topology involving the resonance or transfer of energy between two coils forming a transformer, with one coil acting as a power transmitter (i.e. primary coil) and the other acting as the receiver (i.e. secondary coil). The two coils may for example have a large air gap (e.g. greater than 10 centimeters). At least one capacitor can also be used in parallel and/or series with each of the two coils to enhance the electromagnetic field link between the two coils and allow highly effective power transfer. An alternating current (AC) voltage is generally supplied to the primary coil which induces an alternating voltage in the secondary coil. The induced voltage in the secondary coil is used to charge the battery of the vehicle or electronic device.
Power transfer systems may use electronic switches (e.g. transistors) that are periodically switched on and off at a switching frequency to produce a necessary AC voltage to be supplied to the primary coil. However, many known solutions are sensitive to the switching frequency. In other words, small variations in the switching frequency can cause a large change in the amount of voltage induced in the secondary coil and applied to the battery (i.e. charging voltage). In order to adjust the amount of induced voltage in the secondary coil, the switching frequency may be adjusted. Other problems arise when the switching frequency is adjusted in this manner. For example, as switching frequency increases, the system's overall efficiency decreases due to high switching loss. Existing engineering standards such as SAE J-2954 may also require wireless chargers to operate at specific frequencies. Known power transfer systems may also be limited in their overall efficiency. As such, there is an increasing need for power transfer systems which address the problems of known systems.